During the past year a Phase I trial of recombinant leukocyte interferon in patients with a variety of disseminated cancers revealed that this agent could be administered up to doses of 50 million units/m square i.m. 3 times weekly without unacceptable toxicity. This trial also showed objective evidence of antitumor response (partial remissions) in some patients with non-Hodgkin's lymphoma, breast cancer, chronic lymphocytic leukemia and Hodgkin's disease. Immunologic monitoring of patients receiving this agent failed to reveal any dose-dependent immunologic effect which correlated with tumor response. It was therefore decided to initiate a Phase II efficacy trial of recombinant leukocyte interferon at a maximum tolerated dose, with dose reductions as necessary for unacceptable toxicity. Phase II efficacy trials were initiated in patients with various lymphoproliferative disorders, including non-Hodgkin's lymphoma, chronic lymphocytic leukemia and mycosis fungoides, as well as patients with refractory metastatic breast cancer. Eighteen patients with metastatic breast cancer have been treated with recombinant leukocyte A interferon with 16 patients progressing while on therapy and 2 remaining stable. Forty-six patients with a variety of lymphomas have been treated, with a greater than 50% response rate in favorable histology lymphoma and mycosis fungoides.